The invention relates to membranes having preselected pore types which are made from shaped bodies composed of finely divided carbonaceous materials and binders and wherein the formed bodies are subjected to subsequent heat treatment.
Separating processes employing membranes have obtained great importance in recent processing techniques and have resulted in intensive research activity. Of interest are above all areas where the classic separation methods such as distillation, deep-freeze dissociation and similar processes have failed or are too harsh or too expensive. Thus, membranes have been used for almost 50 years in separating isotopes and during the last few years for obtaining helium from mineral oil and natural gas. Membranes are also used for separation of liquid phases, such as in reverse osmosis, in ultra-filtration and micro-filtration. They are furthermore in use in the nuclear reactor and petroleum industry for the concentration of gaseous materials.
Cellulose-acetate membranes have been used also widely in reverse osmosis processes. They permit molecules with hydrogen bridges, particularly H.sub.2 O to pass by their carbonyl groups while other types of molecules are subject to good retention.
From the German published application No. 1,719,557, it has also become known to employ two layer-dry cellulose acetate membranes for gas separation which have a porous and a non-porous layer. With these membranes the gas separation is effected by active diffusion into the non-porous layer while the porous layer only has protective purposes.
The German allowed application No. 2,321,459 also shows acrylonitrile polymer membranes which have a microporous surface to which are added one or several porous layers with increasing pore dimensions. These membranes are particularly intended for ultrafiltration.
From the German allowed application No. 1,925,582, synthetic membranes are also known of polytetrafluoroethylene which likewise are microporous and can be used for gas separation. A separation is, however, possible only if one face of the membrane is contacted with steam.
There are, furthermore, known from the German published application No. 2,055,559, molecular sieves made of thermally degraded polyphenylene oxides. These materials can also be used in the form of foils and have slot-shaped pores.
The problem of the membrane separation processes is that selectivity and permeability are in competition with each other: a high selectivity lowers the permeability and thus requires an increase of the required membrane surface. Besides, it is desirable to use membranes which are as thin as possible since this reduces the resistance towards movement of the process medium. However, this in turn results in a substantial reduction of the mechanical strength of the membrane. Thus, it is continuously necessary to compromise or to resort to expensive auxiliary materials and steps. Besides, the performance of many membranes is still unsatisfactory.
It is therefore the object of the present invention to provide membranes with preselected pore types which have higher permeabilities than the prior art membranes at the same selectivities and may be made to measure for any specific separation problem as it may arise.
This is important for a use of the membranes at reasonable cost where the problem is the separation of gaseous and liquid mixture and where components have to be concentrated which have a great penetration power. Besides, it is an object to provide membranes of high strength which makes it unnecessary to employ special supporting materials.
Particularly it is of interest to have porous membranes of different pore sizes for use by the coke producing and mineral oil industries.